Implantable medical devices, such as stents, heart valves, bone plates, staples, intrauterine contraceptive devices and the like must meet many requirements to be useful and safe for their intended purpose. For example, they must be chemically and biologically inert to living tissue and to be able to stay in position over extended periods of time. Furthermore, devices of the kind mentioned above must have the ability to expand from a contracted state, which facilitates insertion into body conduits or cavity, to a useful expanded diameter. This expansion is either accomplished by a forced expansion, such as in the case of certain kinds of stent by the action of a balloon-ended catheter, or by self-expansion such as by shape-memory effects.
There are many alloys which display a shape memory effect (SME), known generically as shape memory alloys (SMAs). A widely used metal alloy, particularly for metal applications, is the nickel-titanium alloy, known as "nitinol". Under certain conditions, SMAs can be highly elastic such that they are able to undergo extensive deformation and yet return to their original shape. Furthermore, SMAs possess shape memory properties such that they can "remember" a specific shape imposed during a particular heat treatment and can return to that imposed shape under certain conditions.
The shape memory effect of SMAs results from metallurgical phase transformations. Certain SMAs are characterized by a transition temperature or transition temperature range, above which the predominant metallurgical phase is termed "austenite" and below which the predominant metallurgical phase is termed "martensite". The transformation from austenite (or austenitic state) to martensite (or martensitic state) is termed "martensitic transformation"; the reverse transformation from martensite to austenite is termed as "austenitic transformation". The transformations in fact occur over a range of temperatures and are commonly discussed with reference to M.sub.s and M.sub.f, the start and finish temperatures of the martensitic transformation, respectively, and A.sub.s and A.sub.f, the start and finish temperatures of the austenitic transformation, respectively. Transformation between these two phases is reversible such that the alloys may be treated to assume different shapes or configurations in the two phases and can reversibly switch between one shape to another when transformed from one phase to the other. Occasionally, SMAs display an intermediate phase, between the austenitic and martensitic states, known as the "R phase". Transformation from austenite to the R phase can also be temperature induced, occur at a temperature referred to as "T.sub.R ". The R phase alone does not impart a significant SME (its recoverable strain does not exceed 1%), in contrast to the austenite and martensite states, but rather is a phase strongly affecting the mechanical properties of the alloy and the SME.
Implantable medical devices made of SMA have been known in the art. Sec for example U.S. Pat. Nos. 3,786,806, 4,485,816 and 5,037,427. U.S. Pat. No. 5,242,451 discloses a stent employing a unidirectional SME to conform into an operational shape, from an original basic shape of a smaller diameter. In U.S. Pat. No. 5,562,641, a two-way SME is employed such that the austenitic transformation temperature is above body temperature and the martensitic transformation temperature is below body temperature, whereby the device retains its last conditioned state (e.g. austenite or martensite) at body temperature. U.S. Pat. No. 5,545,210 discloses a stent which is mechanically deformable while in a martensitic state into an operational configuration assumed by it when it is deployed in a tubular organ. The stent of U.S. Pat. No. 5,545,210 exhibits, in its operational configuration, a strain which is on the horizontal plateau of the stress-strain curve of the SMA. U.S. Pat. No. 5,624,508 disclosed a method for the manufacture of shape memory alloy (SMA) device with defined transformation temperature.
The use of stress-induced martensite principle, rather than temperature-induced martensite, has likewise been employed in SMA-based medical devices, e.g. in U.S. Pat. No. 4,665,906. In such devices, austenitic nitinol is deformed to form stress-induced martensite and held in its deformed configuration and martensitic state by a restraining member. The device is introduced into the body in the deformed configuration, where it is released from the restraining member to return to its austenitic state and configuration without any temperature change.